1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereafter, also simply referred to as “photoconductor”) and to a manufacturing method thereof, and more particularly to an electrophotographic photoconductor which comprises mainly a conductive substrate and a photosensitive layer containing an organic material, and is used in electrophotographic printers, copiers, fax machines and the like, and to a manufacturing method of the electrophotographic photoconductor.
2. Background of the Related Art
The basic structure of electrophotographic photoconductors comprises a conductive substrate on which there is disposed a photosensitive layer having photoconductive properties. In recent years, organic electrophotographic photoconductors that use organic compounds as functional components for charge generation and charge transport have been the target of active research and development, and have become ever more widely used in copiers, printers and the like, thanks to their advantageous features, which include material variety, high productivity and stability, among others.
Ordinarily, photoconductors must fulfill the functions of holding surface charge when in the dark, generating charge when receiving light, and transporting the generated charge. Photoconductors include so-called single layer-type photoconductors, which comprise a single-layer photosensitive layer that combines the above functions, and so-called stacked (separate-function) photoconductors that comprise a photosensitive layer in the form of a layer stack in which each layer has a separate function, wherein the layer stack comprises a charge generation layer, that has the function of charge generation upon light reception, and a charge transport layer that has the function of holding surface charge when in the dark and of transporting the charge generated in the charge generation layer upon light reception.
The photosensitive layer is ordinarily formed by coating a conductive substrate with a coating solution in which there are dispersed or dissolved, in an organic solvent, a charge generation material, a charge transport material and a resin binder. Polycarbonate is often used as the resin binder in organic electrophotographic photoconductors, in particular on the outermost surface layer of the photoconductor, thanks to its excellent flexibility, transparency to exposure light, and resistance to friction with paper and toner-scraping blades. Among polycarbonates, bisphenol Z polycarbonates are widely used as the resin binder. Instances of use of polycarbonates as a resin binder are set forth in, for instance, Japanese Patent Application Laid-open No. S61-62040 (Patent Document 1).
Polyarylate resins are also used as the resin binder. West German Patent No. 1200319 (Patent Document 2) sets forth constituent elements such as terephthalic acid, isophthalic acid, succinic acid, adipic acid, sebacic acid and bisphenol, as thermally stabilized dihydroxydiarylalkane materials. Japanese Examined Patent Publication No S48-28800 (Patent Document 3) discloses the feature of using constituent elements such as terephthalic acid, isophthalic acid, adipic acid, sebacic acid, bisphenol A, ethylene glycol or the like, in a method for manufacturing a polyester for easy-sliding films. Japanese Patent Application Laid-open No. S55-58223 (Patent Document 4) discloses constituent elements such as terephthalic acid, isophthalic acid and bisphenol A for enhancing durability against hot and wet thermal aging. Japanese Patent Application Laid-open No. S60-11441 (Patent Document 5) discloses constituent elements such as adipic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, dodecanedicarboxylic acid, phthalic acid, isophthalic acid, terephthalic acid, bisphenol A or the like, in a manufacturing method and a composition for flame-resistant moldings.
In the field of electrophotography, Japanese Patent Application Laid-open No. S64-32267 (Patent Document 6) discloses polybisphenol A-azelate-co-isophthalate as a condensation polymer block in toner compositions. Japanese Patent Application Laid-open No. 2000-352834 (Patent Document 7) discloses a polyester resin that comprises, for instance, terephthalic acid, isophthalic acid, phthalic acid, adipic acid, sebacic acid, azelaic acid or bisphenol A, as a toner image-receiving layer of an electrophotographic image-receiving material for electrophotographic paper or the like. Japanese Patent Application Laid-open No. H4-274434 (Patent Document 8) discloses a bisphenol-based polyester that comprises a bisphenol structure, phthalic acid and an alkylene, as a resin binder for charge transport in a photoconductor, wherein the polyester can be synthesized efficiently, has low melting point viscosity, and has few problematic byproducts. Japanese Patent Application Laid-open No. 2002-23393 (Patent Document 9), the object whereof is to provide an electrophotographic photoconductor having very high light sensitivity and low residual potential, undergoing virtually no residual potential accumulation even after repeated use, exhibiting very low variability as regards charging characteristics and sensitivity, and having excellent stability and durability, discloses a polyester resin that comprises a bisphenol structure, isophthalic acid and terephthalic acid, as a resin binder in a photosensitive layer.
Japanese Patent No. 3953072 (Patent Document 10), the object whereof is to provide an electron photoconductor or the like having excellent solvent cracking resistance and mechanical strength, good anti-electric characteristics, high sensitivity and photomemory, discloses a polyester resin having terephthalic acid, isophthalic acid and alkylene groups as constituent units. Japanese Patent Application Laid-open No. 2005-115091 (Patent Document 11) discloses a polyarylate resin comprising a bisphenol structure, isophthalic acid and terephthalic acid, as a resin binder having high solvent cracking resistance.
However, using a bisphenol Z polycarbonate as a resin binder in an electrophotographic photoconductor was problematic on account solvent cracking and the readiness with which sebum-derived cracks appeared in the photosensitive layer. Solvent cracking is likely to occur due to contact with the solvent of cleaners that are used for cleaning the charging member or the photoconductor. In particular, larger cracks appear on the photosensitive layer when, after cleaning of a contact charging-type charging roller, the solvent does not evaporate completely and remains in contact with the photoconductor.
Recharging and cleaning have become commonplace in photoconductor and cartridges as a response to recycling demands in the wake of growing environmental awareness. Under these circumstances, therefore, it is imperative to solve the problem of solvent cracking. In particular, solvent cracking occurs readily in liquid development processes, since in this case the carrier liquid in which the toner is dispersed comes into direct contact with the photoconductor. A solution to this problem would be thus highly desirable.
To deal with the above problems, Patent Document 1 proposes, for instance, to use a mixture of a bisphenol A polycarbonate resin and a bisphenol Z polycarbonate resin, but this method has proved to be an insufficient solution. The various polyester resins having a bisphenol structure proposed to date have fail to cope sufficiently with the issue of solvent cracking resistance.
It has also been proposed to form a surface protective layer on the photosensitive layer, with a view to, for instance, protecting the photosensitive layer, improving mechanical resistance, and enhancing surface lubricity. However, such surface protective layers as well have failed to avoid the same problem of cracking that bedevils the photosensitive layer.
Under these circumstances, Patent Document 11 discloses a specific polyarylate, as a resin binder, that exhibits unprecedented high solvent cracking resistance. In the current context of ever stronger environmental awareness, however, resin binders for electrophotographic photoconductors should desirably have yet higher solvent cracking resistance.
Thus, it is an object of the present invention to provide an electrophotographic photoconductor and a manufacturing method thereof whereby good images with less cracking occurrence than heretofore can be obtained during recycling of a photosensitive drum and peripheral members thereof, and also in the case of a liquid development process, by improving a resin binder used in a photosensitive layer.